There is a desire in the industry for higher circuit density in microelectronic devices made using lithographic techniques. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. It is known in the art that increasing the numerical aperture (NA) of the lens system of the lithographic imaging tool increases the resolution at a given wavelength. However, increasing the NA results in a decrease in the depth of focus (DOF) of the imaging radiation, thereby requiring a reduction in the thickness of the imaging resist film. A decrease in the resist film thickness can lead to problems in subsequent processing steps (e.g., ion implantation and etching).
In order to overcome these problems, bilayer resists have been developed. Such bilayer resists are generally comprised of a top thin film imaging layer coated on a thick organic underlayer and are patterned by i) imagewise exposure and development of the top layer, and then (ii) anisotropically transferring the developed pattern in the top layer to the thick underlayer and subsequently to the substrate. The top imaging layer contains a suitable refactory oxide precursor such as silicon, boron or germanium that enables the use of oxygen-reactive ion etching (RIE) in the image transfer step.
Additionally, over the past twenty years there has been an industry-wide shift to shorter wavelength exposure systems that also results in a decrease in the DOF. This has been accomplished by reducing the wavelength of the imaging radiation from the visible (436 nm) down through the ultraviolet (365 nm) to the deep ultraviolet (DUV) at 248 nm. Ultra-deep ultraviolet radiation, particularly 193 nm, is now known. See, for example, Allen et al. (1995), “Resolution and Etch Resistance of a Family of 193 nm Positive Resists,” J. Photopolym. Sci. and Tech. 8(4):623–636, and Abe et al. (1995), “Study of ArF Resistant Material in Terms of Transparency and Dry Etch Resistance,” J. Photopolym. Sci. and Tech. 8(4):637–642.
However, as the desired feature size decreases, the resolution capability of even these resists is not sufficient to yield sufficiently small features and the next generation of optical lithography tools under development will employ an F2 157 nm laser as the exposure source. Due to the very poor transparency of conventional resists at this wavelength, new polymer systems will have to be defined. The challenge in developing single and bilayer chemically amplified resists for 157 nm lithography is in achieving suitable transparency in polymers that have acid-labile functionalities or crosslinking groups and thereby convert to materials that are either soluble, when used as a positive resist, or insoluble when used as a negative resist, in industry standard developers.
Studies, such as Kunz et al (1999), Proc. SPIE 13:3678 and Crawford et al (2000), Proc. SPIE 357:3999 have identified two main classes of polymeric materials that are sufficiently transparent at 157 nm to be useful in single and bilayer resists; fluorocarbon polymers, and polysiloxanes (including polysilsesquioxanes). In the case of bilayer resists, siloxanes and silsesquioxanes are particularly advantageous because of their high silicon content. Specifically, polysilsesquioxanes will be ideal candidates for 157 nm bilayer resist development, as well as single layer resist development, because generally they have higher Tg than the polysiloxanes.
Fluorocarbon polymers, such as polymers prepared from trifluoromethyl-substituted acrylates have been described previously. See, for example, Ito et al. (1981), “Methyl Alpha-Trifluoromethylacrylate, an E-Beam and UV Resist,” IBM Technical Disclosure Bulletin 24(4):991, Ito et al. (1982) Macromolecules 15:915–920, which describes preparation of poly(methyl α-trifluoromethylacrylate) and poly(α-trifluoromethylacrylonitrile) from their respective monomers, and Ito et al. (1987), “Anionic Polymerization of α-(Trifluoromethyl)Acrylate,” in Recent Advances in Anionic Polymerization, T. E. Hogen-Esch and J. Smid, Eds. (Elsevier Science Publishing Co., Inc.), which describes an anionic polymerization method for preparing polymers of trifluoromethylacrylate. Willson et al., Polymer Engineering and Science 23(18):1000–1003, also discuss poly(methyl α-trifluoromethylacrylate) and the use thereof in a positive electron beam resist.
Photoresists comprised of silsesquioxane polymers have also been previously described. See, for example, U.S. Pat. No. 6,087,064 to Lin et al., U.S. Pat. No. 5,385,804 to Premlatha et al., U.S. Pat. No. 5,338,818 to Brunsvold et al., and U.S. Pat. No. 5,399,462 to Sachdev et al., which disclose the use of aryl or benzyl substituted polysilsesquioxanes in photo resists. However, none of these references disclose utility of fluorocarbinol and/or fluoroacid functionalized polysilsesquioxanes in 157 nm single and bilayer resist applications.